Low alloy steel containing titanium



Patenteci Jan. 31, 1950 UNITED STATES PATENT OFFICE No Drawing. Application August 14, 1942, Serial No. 454,835

Claims.

The present invention relates to improvements in ferrous alloys which may be classed as lowalloy high-strength structural steels, which have been developed especially to meet the various structural requirements better than is possible with plain carbon steels or with the simpler low nickel, low silicon, medium manganese or copperbearing grades.

These steels must possess an inherently high yield strength (elastic limit) and ultimate (breaking) strength and, desirably, a moderately wide plastic range between the two. They must be suitable for fabrication by the usual methods and for use in structures without special treatment. Ductility or formability must be adequate to permit them to withstand moderately severe cold forming or bending in certain applications. Weldability, by which is meant both the welding operation itself and the withstanding of this operation by the steel without embrittlement or other harmful effect, is-another quality which is necessary for many applications. Increased resistance to corrosion is a property possessed by all of these steels in varying degrees but all are superior to plain carbon or simple alloy grades.

A high elastic limitor yield strength permits a correspondingly high stress or load. Structural economies may thus be efiected either by increasing the load on a given section or, if more desirable, reducing the size of the section used for a given load.

In rigid structures which require little or no pro-forming or bending, ductility (i. e., the extension of the material before breaking) is of relatively minor importance. However, in fabricating by cold forming processes, which involve stressing the material beyond its elastic limit, so that it takes a permanent set, ductility is Very important. While it is essential that such steels possess adequate ductilility for cold forming, it has been found that ductility, as measured by the conventional tensile test, doesnot always reflect accurately the ability of the steel to withstand actual forming operations. This ability is referred'toherein as formability. While theonly accurate measurement of formability is the actual formation itself, certain laboratory tests have proven better indicators'of this quality in performance than the tensile test. Among these are the bend test and the cupping test, which are used principally (the latter exclusively) on sheet steel. It has been found that difierent steels possessing approximately equivalent ductility as measured by tensile tests, may differ widely in formability as judged by the bend and cupping tests, and as verified in actual forming operations.

In addition to high yield and tensile strength, the permitting of weight reduction in design, adequate ductility, formability and improved corrosion resistance, another important property of the steels under discussion is weldability. By this term is meant not only the ability to be welded readily and satisfactorily by the usual commercial methods, but also the ability of the steel adjacent to the weld to retain, after welding, substantially all of its original desirable properties. Some of the low alloys steels are deficient in this regard, in that they tend to become brittle, i. e., lose their ductility and formability in a region adjacent to and a short distance away from a Weld.

A further disadvantagefound on some of these alloys, particularly objectionable on steel sheets, is their tendency to hydrogen embrittlementduring pickling in acid solution to remove hot rolling scale or oxides. Such sheets suffer additional embrittlement during galvanizing operations, in which the sheets are passed through a pot containing molten zinc at approximately 800-840" F. Since the galvanizing operation in usual practice is preceded by a pickling treatment, the total embrittlement caused by the two operations may be so great as to preclude the use of the steel for applications involving moderately severe cold forming.

A large variety of alloys is included in the general classification of low-alloy high-strength structural steels described above. Such alloys contain generally two or moreof the following elements in greater quantity than normally found in plain carbon structural grades: copper, nickel, chromium, manganese, silicon, phosphorus, molybdenum. Carbon is kept low to avoid its embrittling effect, and also because it combines with certain of the above elements, principally chromium, to form undesirable and embrittling carbides. Upper limits are likewise placed on several other elements, principally copper, silicon and phosphorus, because of their embrittling eiiect if contained in too large amount. 7

The alloy steel of the present invention belongs to that type which is known as the copperphosphorus-chromium type. The usual limits of the constituents of this alloy are given in a table presented below. This alloy steel has been found to be particularly susceptible to embrittlement following welding or galvanizing operations.

In accordance with the present invention, itis found that steels of this type can be greatly improved by the addition of titanium to an extent to analyze 0.02% to 0.30% in the steel. Such additions not only produce marked increase in the yield strength and ultimate strength, but simultaneously effect great improvement in formability, the ductility remaining essentially unaffected. Further, such additions result in the substantial elimination of embrittlement in these steels as a result of welding or galvanizing.

The following table shows the range of composition of the steels which have been tested. It will be understood that the column marked Average is not meant to-be the mathematical average of the minimum and maximum values, but is the composition which approximates the usual or average content in actual practice.

In the foregoing composition the elements sulphur, silicon and nickel may or may not be added. If not purposely added, each of each elements would be present as a residual element, that is, an element which is not a main alloying element, but results from the addition of alloys normally present in the furnace charge as unavoidable components thereof. Titanium was added to steels of the above general analysis in amounts such as to give a final content thereof from about 0.02 to 0.30%, resulting in the beneficial effects described hereinafter.

An an illustration of the improvement to this alloy steel resulting from the addition of titanium in the amounts above cited, the following test was conducted:

Strip samples were prepared as follows: Strip hot-rolled to .074" thick, box-annealed at approximately 1250 F., cooled in a reducing atmosphere, and lastly temper-rolled.

Following such treatment, the titanium-bearing steel was found by tensile test to have a yield strength approximately 20,000 p. s. i. above normal and an ultimate (breaking) strength approximately 10,000 p. s. i. above normal, while the ductility was slightly below normal (normal values being taken as: Yield strength 51,000 p. s. i., ultimate strength 72,000 p. s. i., elongation 28 per cent in 2") Bend tests were made on samples of the sheets processed as above and also following a quench in cold water from 1750 F. These tests were made on 2" wide samples both parallel to and transverse to the direction of rolling of the strip by bending 180 flat over a radius equal to onehalf the thickness of the sheet and flattening the tests with a sledge hammer. Such tests on the as-finished samples were passed satisfactorily by both the regular steel (without titanium) and the titanium-bearing steel. On the quenched samples, however, the steel made without titanium failed, while the titanium-bearing samples withstood this test successfully.

So-called handkerchief bend tests, consisting of an additional 180 fiat bend test transverse to and superimposed upon the original fiat bend test, were made upon samples from the asfinished sheets. In this test, the regular steel 4 (without titanium) failed by cracking, while the titanium-bearing steel passed successfully.

Strip samples were additionally prepared by a modification of the treatment outlined above, in which a normalizing anneal and pickling treatment were inserted between the hot-rolling and box-annealing operations. Following such treatment, sheets of regular composition (without titanium) failed on bend tests of quenched samples and handkerchief bends of as-finished samples. The titanium-bearing strip passed these tests successfully.

As a further severe test, titanium-bearing strip, hot-rolled only, without either box-annealing or normalizing, was tested by single flat bend and handkerchief bend tests. These tests were passed successfully with only fine cracks showing, and were superior to similar tests made on regular composition strip (without titanium) which had received both normalizing and box-annealing treatments.

To develop adequate formability on regular composition (no titanium) steel, so that such steel will be suitable for many applications, it is necessary to give the strip sheets both a normalizing anneal and box-anneal treatment. Since adequate formabilit can be obtained with the titanium-bearing steel with only a box-anneal operation (and for many uses no anneal), this permits the elimination, and resultant savin in cost of the normalizing anneal and following pickling operation (and, sometimes, also of the box anneal) The above tests indicate the following improvements and advantages of the titanium-bearing grade in comparison with the copper-phosphoruschromium type of steel to which no titanium addition has been made.

1. Markedly higher yield strength and ultimate strength with adequate ductility on strip sheets which have been treated as follows: Strip hot-rolled, box-annealed, deoxidized, and temperrolled.

2. Marked improvement in formability, as judged by handkerchief bend tests, of sheets processed as in (1) above, as well as when a normalizing and pickling treatment is inserted between hot-rolling and box-annealing.

3. Marked improvement in formability, as judged by fiat-bend tests, of strip sheets quenched in cold water from 1750 F. following treatments given in (1) and (2) above.

4. Elimination of heavy loose scale frequently produced on the surface of strip sheets of regular anaylsis (without titanium) processed as in (1) and (2) above.

It has been found that the presence of titanium in amounts of approximately 0.02 to 0.30% in the copper-phosphorus-chromium type of steel also imparts superior formability to sheets hot-rolled on the conventional two-high mill. This improvement enables sheets for many purposes to be shipped as hot-rolled, eliminating the expense of the annealing operation and, probably, of one pickling and one flattening operation. The formations to which sheets may be subjected in fabricating operations have been found to cause a high percentage of breakage in sheets of this grade, without titanium, even when such sheets have been subjected to a normalizing anneal, whereas breakage in the titanium-bearing sheets is substantially eliminated under such processing.

The difference in formability referred to above is even more marked when such sheets have been pickled and alvanized. The embrittling effect produced by hydrogen evolved in pickling is a function of the time required for this operation. Normally about 30 minutes are required to pickle sheets of this grade. The time required may be somewhat less than this, or, if the scale is unusually adherent, times as long as 45 minutes or even an hour or longer may be required.

To illustrate the embrittling effect due to pickling for various lengths of time, followed by galvanizing, the following test was made:

Sheets of 16 ga. (.062") and 13 ga. (.093") of regular composition (without titanium) and titanium-bearing steel, both in the as hot-rolled and fas normalized condition, were pickled in 9% sulfuric acid at 120 F. for varying lengths of time as follows: 15 minutes, minutes, minutes, one hour. The sheets which had been thus pickled were then galvanized-coated in the usual manner. Olsen ductility tests, with a 1" diameter ball, were made on samples from near the edge and near the center of each sheet before and after each of these operations. Such tests, being made on small samples, measure local variations in ductility which do not always accurately reflect the formability of the sheet as a whole, but may, nevertheless, be used in conjunction with other tests to form a basis for judging this quality. For moderately severe forming operations, an Olsen ductility value expressed in inches penetration of the sheet before fracture of .400" on most tests is desirable, with occasional tests falling as low as .375".

On the above basis of judging Olsen ductility tests, all of the 16 ga. sheets of regular analysis (without titanium) failed, all testing below .375". Three out of seven of the 13 ga. sheets of regular composition (without titanium) tested below .400 and were thus on the low side of the acceptability'range. All of the 16 ga. and 13 ga. sheets of titanium-bearing steel passed this test, only. one test of one sheet being below .400" (.389"). As may be seen from the average result of all of such tests on the titanium-bearing sheets given in the tabulation below (.460") these sheets generally tested farin excess of minimum requirements.

The average results of the Olsen ductility tests are given in the following table:

It will be seen from the above that the steel made without titanium loses ductility in both the pickling and galvanizing operations, while the titanium-bearing grade regains durin the galvanizing operation a portion of the ductility lost during pickling. The final results indicate the regular type of steel (without titanium) to be unsatisfactory for moderately severe forming, while the titanium-bearing grade is well above 6 the minimum requirements for such service (.400").

In addition to the Olsen ductility tests described above, flat bendtests and handkerchief bend tests were made of the various sheets. The regular sheets (without titanium showed cracks on the flat bend tests of 16 ga. sheets, while the 13 ga. sheets broke in this test. The 16 ga. and

13 a. titanium-bearing sheets not only withstood.

lar steel (without titanium), the 16 ga. sheets cracked, while the 13 ga. sheets. broke. On the titanium-bearing sheets, both the 16 ga. and 13 a. sheets passed the test without developing any cracks.

As a result of the above tests, as well as forming tests on 13 ga. galvanized sheets in which such sheets were subjected to a moderately severe drawing operation in a fabricating shop, the following improvements are claimed for titanium-bearing alloy of the copper-phosphorus-chromium type over this grade made without titanium.

1. Superior formability of hot-rolled sheets.

2. Embrittlement caused by galvanizing is substantially eliminated.

3. Embrittlement caused during the pickling operation is partially relieved during subsequent galvanizing,

4. Embrittlement of the sheet adjacent to a weld is substantially eliminated.

It is not known definitely why the addition of titanium imparts the desired effects of relieving embrittlement, but the following considerations are ofiered as being a possible reasonable explanation. In the process of manufacture of corrosion resistant low-alloy high-strength structural steel containing about 1.0% chromium and 0.50% silicon, for example, ferro-silicon is added in the open hearth furnace first, the silicon content being sufiicient to kill the steel, the product of deoxidation (silica) going largely into the slag. Ferro-chromium; being more costly but a stronger deoxidizer then is added, principally in the furnace after the addition of the silicon, in order to avoid high. chromium losses. Thus the steel is thoroughly killed in the furnace, with a large, excess of deoxidizer content. Aluminum, the, most powerful deoxidizer in general use, is added in the ladle to carry the deoxidation further than is possible with less powerful deoxidizers. Thus, in the presence of an excess of free aluminum, a more powerful deoxidizer than titanium, the latter would have no deoxidizing effect. Further, the reaction products formed by the titanium addition necessarily would remain largely in the steel as the latter is fully killed and quiescent, that is to say, the steel is not effervescent as is rimming steel, for instance.

With reference to such rimming steels, it may be noted that the effect of copper and nickel on such steels is similar, both tending to strengthen the ferrite grains with retentionof fairly good ductility. Titanium has no effect on either of these elements.

Therefore, chromium remains as the only common residual alloy component of rimming steels which may be influenced by titanium additions,

Since residual chromium has been present during the entire melting of such steels, and also during the working and pouring of the heat, it is most likely to be in solution in the ferrite grains in a manner similar to copper and nickel, as any oxide has had ample opportunity to escape to the slag. Thus, due to the favorable opportunities for escape, the chromium content of low carbonrimmed steels cannot exceed .03 to .04%. But in the event that some chromium might remain in the steel in the form of iron-chromium oxide FeO-CrzOz, it may be noted that chromium oxide has a'higher heat of formation than titanium oxide, and, therefore, it is more refractory than titanium oxide, so that the titanium is incapable of reducing the said iron-chromium oxide.

Therefore,-it will be seen that there is no effect on the residual alloys of low carbon rimming steel from titanium additions. The action of titanium added to such steels is a deoxidizing one, the products of deoxidation being largely removed from the ingot by the rimming action of same. Such action is quite different from that of titanium on low-alloy high-strength structural steels described above, which is not a deoxidizing action and in which the reaction products remain in the steel.

Thus, it appears probable that the benefit of titanium additions to low-alloy high-strength structural steel results from the combination of said titanium with sub-microscopic precipitants normally present in such grade, which, uncombined, result in embrittlement of the steel following hot-rolling, galvanizing or welding operations, but which, in combination with titanium, are incapable of producing such harmful effects.

This application is a continuation-in-part of application, Serial No. 404,721, filed July 30, 1941, now abandoned, and entitled Steel.

1 claim:

1. A low-alloy high-strength structural steel composed of from substantially 0.02% to 0.20% carbon, 0.10% to 1.50% manganese, 0.02 to 0.20% phosphorus, sulphur, residual to 0.05%, 0.02 to 1% silicon, 0.02 to 1% copper, 0.30% to 2% chromium and 0.02 to 30% titanium, the balance being essentially iron.

2. A low-alloy high-strength structural steel composed of the following components in substantially the percentages indicated: carbon 0.10%, manganese 0.40%, phosphorus 0.12%, sulphur 0.03%, silicon 0.5%, copper 0.40%, chromium 1.10%, nickel and titanium .10%, the remainder being essentially iron.

3. A low-alloy high-strength structural steel composed of from substantially 0.02 to about 0.20% carbon, from about 0.10 to about 1.50%

manganese, from about 0.02 to about 0.20% phosphorus, sulphur, from residual amounts to about 0.05%, from about 0.02 to 1% silicon, from about 0.02 to 1% copper, from about 0.30 to 2.00% chromium, and from about 0.02 to about 0.30% titanium, the balance being essentially iron, the said steel being characterized by remaining substantially free from brittleness when subjected to treatments normally embrittling the steels of this type free from titanium.

4. An alloy steel of low carbon content between 0.02 and 0.20% carbon and containing as the essential alloying constituents 0.02 to 1.00% copper, 0.30 to 2.00% chromium, 0.02 to 0.20% phosphorus, and from 0.02 to 0.30% titanium, the balance being essentially iron, and characterized by high tensile strength combined with ductility corrosion resistance, and weldability, and by absence of embrittlernent, which embrittlement occurs in a steel containin no titanium but otherwise of the same composition when the said steel is subjected to hot rolling, pickling, and galvanizing or welding.

5. A low alloy steel containing 0.05 to 0.14% carbon, 0.07 to 0.18% phosphorus, 0.30 to 0.50% copper, 0.10 to 0.30% manganese, 0.50 to 1.50% chromium and 0.03 to 0.18 titanium, the balance being substantially all commercial steel.

EDGAR MARBURG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Comstock Apr. 28, 1942 OTHER REFERENCES Bulletin No. 2: A Report of Experimental Research Investigation upon Corrosion and Mechanical Influences of Alloying Elements in Low Alloy Steels, pages 40, 41, and chart R. Published prior to August 23, 1940 by the Monsanto Chemical Company, St. Louis, Mo.

Metals Handbook, 1939 edition, pages 479 to 483, 559, and 560. Published by the American Society for Metals, Cleveland, Ohio.

Titanium and Its Use in Steel, pages 28, '70, and '71. Published in 1940 by the Titanium Alloy Mfg. Co., New York.

Phosphorus-Iron Alloys, Bulletin No. 1, page 24. Exploratory Research upon Influence of Phosphorus in Low Alloy Steels. Published prior to August 23, 1940, by the Monsanto Chemical Co., St. Louis, Mo.

Number 

1. A LOW-ALLOY HIGH-STRENGTH STRUCTURAL STEEL COMPOSED OF FROM SUBSTANTIALLY 0.2% TO 0.20% CARBON, 0.10% TO 1.50% MANGANESE, 0.02% TO 0.20% PHOSPHORUS, SULPHUR, RESIDUAL TO 0.05%, 0.02% TO 1% SILICON, 0.02% TO 1% COPPER, 0.30% TO 2% CHROMIUM AND 0.02 TO .30% TITANIUM, THE BALANCE BEING ESSENTIALLY IRON. 